1. Field of the Invention
This invention relates to a rabble arm, and more particularly, to a rabble arm having an oval or elliptical cross-sectional configuration which allows the arm to be subjected to increased bending stresses within the heated interior of a multiple hearth furnace as the arm is operated to convey material within the furnace.
2. Description of the Prior Art
It is well known to utilize a high temperature furnace such as a multiple hearth furnace to heat or roast many types of material. The multiple hearth furnace includes a cylindrical outer shell with a plurality of vertically spaced hearths positioned therein. A vertical shaft extends through a central opening in each hearth. A plurality of rabble arms are connected to the vertical shaft in cantilever fashion, and the vertical shaft is rotated to allow the plurality of rabble arms to rotate within the interior of the furnace. The rabble arms hold a plurality of rabble teeth which contact the material being heated within the furnace, and the rabble teeth move the material positioned on a given hearth either towards the central opening or outside wall of the furnace.
Each rabble arm has a hollow interior which is connected to a hollow interior of the vertical shaft. A cooling medium, such as air under pressure, is passed through the hollow interior of the vertical shaft and is circulated through the hollow interior of each rabble arm to reduce the temperature of the arm. Cooling the interior of the rabble arm allows the arm to withstand higher bending stresses, since the limiting creep stress of the arm is inversely proportional to the temperature experienced by the arm. Since each rabble arm is connected to the furnace vertical shaft in cantilever fashion, each arm must be capable of supporting its own weight and the weight of the rabble teeth connected thereto at elevated furnace temperature. One type of failure of the rabble arm at elevated temperatures is predominantly due to sagging or bending vertically under its own weight and eventually breaking.
Rabble arms used in multiple hearth furnaces have heretofore been sand cast in one piece and have a hollow interior portion with a D-shaped cross-section turned 90.degree. so that the planar surface of the arm lies horizontally. Rabble teeth are attached to the arm by either a dove tail or clevis-type connection. However, sand cast rabble arms having a D-shaped cross-section are relatively expensive to form, are extremely heavy, and are subject to all the manufacturing limitations of foundries such as core shift and shrink. These limitations all tend to weaken the sand cast arm and cause the arm to sag under its own weight over a period of time within the furnace. As the rabble arm sags, cracks develop between the air cooled interior and the outer wall of the arm to cause cooling air to leak into the furnace. Eventually, because of the progressive sagging of the rabble arm under its own weight the arm will be removed and replaced.
More recently, rabble arms have been made which utilize a circular, centrifugally cast alloy tube which is welded to a sand cast socket end. Rabble teeth holders are also welded longitudinally on the outer surface of the arm. The circular cross-sectional rabble arm construction offers some quality benefits, such as uniform tube wall thickness. However, the circular cross-sectional arm sacrifices strength due to the inherently weak circular cross-section of a tube.
U.S. Pat. No. 1,076,297 discloses an agitating or rabble arm for use in multiple hearth or roasting furnaces. The rabble arm has a center portion which is supported by a vertically extending center shaft within the furnace. The rabble arm extends radially from the center shaft on opposite sides of the shaft, and has a hollow interior for receiving a cooling medium. The portions of the rabble arm which extend radially from the center shaft each have a rectangular cross-sectional configuration which decreases from a location adjacent the center shaft to the end of the arm.
U.S. Pat. No. 1,444,209 discloses an ore roasting furnace which includes a plurality of rabble arms each having a D-shaped cross-sectional configuration turned 90.degree. so that the planar surface of the arm lies horizontally and connected to a rotating shaft which extends vertically through the center of the furnace.
U.S. Pat. No. 1,687,935 discloses a rabble arm having a generally circular cross-sectional configuration which is connected to a rotating center shaft of a multiple hearth furnace. U. S. Pat. No. 2,332,387 discloses a rabble arm for use in a multiple hearth furnace which has a generally D-shaped cross-sectional configuration turned 90.degree. .
U. S. Pat. No. 3,905,757 discloses an apparatus and method for distributing solid material over a plurality of vertically spaced hearths in a multiple hearth furnace. A rotatable center shaft extends vertically through the center of the furnace to pass through each hearth. Alternate hearths have drop holes disposed toward the center shaft and the other hearths have drop holes disposed towards the outer periphery thereof. A plurality of rabble arms are connected to the rotatable center shaft and have rabble teeth secured thereto which contact the solid material on each hearth. The rabble arms are illustrated as having a generally D-shaped cross-sectional configuration turned 90.degree. so that the planar surface of the arm lies horizontally.
While the prior art suggests utilizing a rabble arm having either a circular, D-shaped or rectangular cross-sectional configuration, there is a need for an improved rabble arm which is connected to a rotating center shaft of a multiple hearth furnace. The improved rabble arm has an outside surface of substantially oval cross-sectional configuration to increase the rabble arm's resistance to sagging or bending vertically relative to a longitudinal axis of the arm when subjected to elevated temperatures over an extended period of time.